Extrusion heads for continuous extrusion forming of continuous plastic elements having specific cross-sectional shapes are well known. Such extruded elements may include, for example, pipes, rods, moldings, tubings, and the like.
In a typical prior art extrusion system, solid pellets of the thermoplastic material to be used are fed into a progressive-screw extruder wherein the pellets are liquefied under high pressure and are injected into an extrusion head. Such injection may be made axially of the extrusion head, known in the art as “inline,” or at an angle, typically 90°, to the axis of the head, known in the art as “crosshead.” Except when coating highly flexible core materials such as wire, the coating of a sheath layer onto a core stock requires passing the core stock axially through a die and injecting the molten polymer into the die head in a crosshead relationship.
In a typical prior art extrusion crosshead, a generally cylindrical body element concentrically surrounds a generally cylindrical mandrel, a first annular flow space being provided therebetween. Molten polymer injected orthogonally from a screw extruder enters an annular reservoir provided in either the body element or mandrel and then flows from the reservoir along the annular flow space. Contiguous with the annular flow space is a conical flow space, formed between a conical choke ring and a conical portion of the mandrel, wherein the diameter of annular flow is decreased and the velocity of flow is increased. Downstream of the conical flow space is a second annular flow space formed between a second cylindrical region of the extruder body and a second cylindrical region of the mandrel. This flow space leads into a flow shaping region formed between an extrusion die and an extrusion tip, from whence the formed shape is extruded.
When it is desired to form a coating on a core element, the mandrel and extrusion tip are provided with an axial passage through which the core element is passed as extrusion proceeds.
When it is desired to provide a spiral element in a coating, the extrusion die may be made rotatable of the extrusion body.
Several problems exist in prior art extrusion heads having rotatable dies.
First, it has been found to be difficult to provide a rotatable seal to prevent leakage of molten polymer from the head between the stationary and rotating components. Typically, such leakage causes continuous polymer buildup on the outside of the head, eventually requiring shutdown of the process to clean and restart.
Second, polymer may leak into the bearings, causing failure of the head.
Third, the extrusion die and tip must be heated externally to prevent freeze-up at the start of operation.